(int )_(-infty )^+infty (x)^2(e)^-a(x^2)dx=dfrac (1)(2)sqrt (dfrac {pi )({a)^3}} . (int )_(-infty )^+infty (x)^4(e)^-a(x^2)dx=dfrac (3)(4)sqrt (dfrac {pi )({a)^5}}

(int )_(-infty )^+infty dfrac (dx)({x)^2+2x+2}..

(2) (int )_(-infty )^+infty dfrac (dx)({x)^2+2x+2}

()-|||-(2) (int )_(-infty )^+infty dfrac (2x)(1+{x)^2}dx=lim _(aarrow +infty )(i

(int )_(e)^+infty dfrac (dx)(x{ln )^2x}= __

[题目]-|||-(int )_(2)^+infty dfrac (dx)((x+7)sqrt {x-2)}= __

(int )_(0)^+infty dfrac (x{e)^-x}({(1+{e)^-x)}^2}dx= ()-|||-__

计算(int )_(0)^+infty dfrac (dx)({x)^2+4x+3}的值是A (int )_(0)^+infty dfrac (dx)({x)^

[例3] (int )_(0)^+infty dfrac (x{e)^-x}({(1+{e)^-x)}^2}dx

19、单选 反常积分 (int )_(0)^+infty dfrac (1)(1+x+{x)^2}dx= __-|||-(3分-|||-A) .dfrac (s

[题目]计算 (int )_(1)^+infty dfrac (arctan x)({x)^2}dx